This disclosure relates to methods and apparatus useful for discriminating between signals on the basis of the spatial orientations of the normals to their traveling wavefronts. Since the methods and devices disclosed herein may operate on signals that need differ only in the directions of propagation of their wavefronts, and may otherwise be of arbitrary relative strengths, waveforms, spectral or modulation characteristics, these methods and devices are particularly adapted to separate mutually interfering signals, as in multipath transmission, multitarget radar returns, electromagnetic jamming, and multiplex transmission.
Prior art techniques for discrimination between signals on the basis of the directions of arrival or of departure of their wavefronts are based on antenna pattern synthesis by aperture design or/and combining the simultaneous outputs of separate antennas in such ways as, in effect, to create directional mainlobes or/and directional nulls. Those prior techniques, while proven effective and often affordable for directions that differ by several degrees or more, they tend to become generally prohibitively costly when designed to resolve smaller directional differences, and when designed to adapt to changes in directions of wavefronts or/and to more than a few simultaneous wavefronts. These remarks are particularly applicable to operating frequencies in the lower microwave and below bands.
The techniques disclosed herein provide a means for discriminating between signals differing in the directions of propagation of their wavefronts by imposing on each signal a modulation that depends, in modulation level and/or modulation phase or time reference, upon the relative orientation of the normal to the signal wavefront. It is well-known from basic modulation theory that modulation of a carrier always results in a redistribution of a fixed amount of signal power over a frequency spectrum that is always wider than the original signal or carrier spectrum width, thus reducing the signal power density within that original bandwidth. Accordingly, with each signal thus modulated with a modulation level proportional to the direction co sine of its wavefront, the power spectral densities of the differently directed signals will be reduced within their original bands in amount determined by the square of the respective direction cosine. The discrimination between signals could then be based on which signal has its power spectral density within a specified band reduced more or less than which.
Accordingly, it is an object of the present invention to provide a spectrum spreading technique which diverts a portion of the signal power or energy from within the original signal bandwidth to outside of that bandwidth, the proportion thus diverted being dependent on the square of the direction cosine of the normal to the signal wavefront.
It is another object of the present invention to provide a means for distinguishing between signals by causing each to have a fraction of its power or energy, determined by the direction cosine of its wavefront, to be diverted into a more widely spread frequency range.
It is yet another object of this invention to provide a technique for causing a fraction of a signal's power, dependent on the direction cosine of its path ray, to be diverted into a new component signal whose carrier reference is of the same frequency as the original carrier reference but is orthogonally phased relative to the original carrier reference, and whose power or energy spectral density is spread out over a wider frequency range than the original signal.
It is yet another object of this invention to provide a means for separating two signals arriving from, or radiated to, different directions by causing none of the power or energy of one to be dispersed outside of its original bandwidth, while at the same time causing all of the power or energy of the other to be diverted to a spectrally spread replica of the original signal extending over a bandwidth wider than that of the original signal.
It is yet another object of this invention to provide a means for multiplexing signals on the basis of the relative directions of the normals to their wavefronts, which may be referred to as direction-division multiplexing.
It is yet another object of this invention to provide a means for demultiplexing signals on the basis of the relative directions of the normals to their wavefronts, which may be referred to as direction-division demultiplexing.
It is yet another object of this invention to provide a method for picking up signals or radiating them that causes a digital modulation to be imparted to some parameter (e.g., amplitude, phase, frequency or polarization) of each signal, such that the modulated parameter shift due to said imparted modulation is determined primarily by the direction cosine of the signal wavefront.
It is yet another object of this invention to provide a method for picking up signals or radiating them that causes an analog modulation to be imparted to some parameter (e.g., amplitude, phase, frequency or polarization) of each signal, such that the modulated parameter deviation due to said imparted modulation is determined primarily by the direction sine of the signal wavefront.
It is yet another object of the present invention to provide a means for radiating or receiving time-shifted replicas of the same digitally code-modulated signal such that these replicas combine in perfect coincidence in time or phase in only one particular radial direction and noncoincidence in all other radial directions.
These and other objects and features of this invention will become apparent from the claims, and from the following description when read in conjunction with the accompanying drawings.